currently in most cases a tolower will do the job, and even for fairly strict Unicode conformance a tolower(toupper(c)) will work.
You do know that toupper(c) is conceptually broken, don't you?
There is at least one character (U+00DF Latin Small Letter Sharp S), whose upper case equivalent in German is TWO characters. Of course, in the case of regular expressions, it might well be acceptable or even best to treat scharfes-es as equivalent to "ss" and then - however I always get nervous when I see people using tolower/toupper in I18N contexts.
I know, and I mentioned this in my last post. However, note that we don't even have an API that can handle this, certainly not std::ctype!
(Note that tolower is worse, to convert a string to lowercase in German you need a dictionary to decide whether to use "ss" or scharfes-es - and I think that there is a case where you need to interpret the text to decide which word it is! Oh, and finally, this all assumes that the proposed German spelling reforms don't happen because I think they largely eliminate the scharfes-es :-)
Nice to know that English isn't the only "illogical" language out there ;-) John.